The invention relates to a switching device for energizing a lamp, comprising
input terminals which are to be connected to the poles of a supply-voltage source,
a DC-AC converter for generating a high-frequency lamp current of frequency f from a supply voltage supplied by the supply-voltage source, which DC-AC converter comprises
a branch including a series arrangement of a first and a second switching element, respective ends of which are coupled to the input terminals,
a control circuit including a microprocessor for rendering the first and the second switching element alternately conducting and non-conducting, which control circuit is provided with a dim circuit having
a timer for adjusting a first time interval, during which the first switching element is conducting, and for adjusting a second time interval, during which the second switching element is conducting.
The invention also relates to a control circuit for use in such a switching device.
A switching device of the type mentioned in the opening paragraph is known from WO 99/01013. As the control circuit comprises a microprocessor, many different functions of the switching device can be implemented in the control circuit, which, in spite of that, can be embodied so as to be comparatively small. For example, the known switching device enables the luminous flux of the lamp to be adjusted at a desired value by influencing the conduction times of the switching elements. To achieve this, software is employed enabling a number to be set in the timer by the microprocessor, the value of said number being a measure of the desired conduction time of a switching element or a measure of the desired length of a time interval during which the switching elements are both non-conducting. Subsequently, the contents of the timer is reduced to zero by the microprocessor in a number of steps that is equal to the number set in the timer. When the value of the number in the timer has been reduced to zero by the software, one of the switching elements is rendered conducting or non-conducting. A drawback of this method of dimming a lamp energized by means of the switching device resides in that the resolution of the time intervals adjusted by means of the timer is determined by the time needed by the microprocessor to execute a command originating from the software. Unless use is made of a very expensive microprocessor, this means in practice that the resolution is comparatively poor. As a result, the number of levels of the luminous flux of the lamp that can be adjusted by means of the control circuit is comparatively small. A further drawback of the known switching device resides in that, during stationary operation, the microprocessor almost continuously determines conduction times of switching elements and/or time intervals during which both switching elements are non-conducting. As a result of this further drawback, the capacitance that remains to execute other functionalities, such as control functions etc, is very limited.
It is an object of the invention to provide a switching device by means of which the luminous flux of a lamp energized using the switching device can be adjusted at a comparatively large number of values, while, apart from controlling the conduction times of the switching elements, the microprocessor is capable of carrying out a large number of other functions during stationary operation.
To achieve this, a switching device of the type mentioned in the opening paragraph is characterized in accordance with the invention in that an input of the timer is connected to a signal generator for generating a signal whose frequency is higher than the clock rate of the microprocessor, and in that the timer is provided with a reset circuit for resetting the timer with a frequency 2*f, and in that the dim circuit further comprises
a first comparator, a first input of which is coupled to an output of the timer, and a second input is coupled to an output of a first reference circuit for generating a signal which is a measure of a desired value of the first time interval, and an output of which comparator is coupled to a control electrode of the first switching element,
a second comparator, a first input of which is coupled to the output of the timer, and a second input is coupled to an output of a second reference circuit for generating a signal which is a measure of a desired value of the second time interval, and an output of which comparator is coupled to a control electrode of the second switching element.
In a switching device in accordance with the invention, the contents of the timer is changed with the frequency of the signal generator. This frequency is independent of the clock frequency of the microprocessor and can be selected so as to be much higher than said clock frequency. This comparatively high frequency of the signal from the signal generator results in a comparatively high resolution of the time intervals determined by means of the timer. As a result of this high resolution, the luminous flux of the lamp can be set to a comparatively large number of values.
In a preferred embodiment of a switching device in accordance with the invention, an input of the first reference circuit and an input of the second reference circuit are coupled to the microprocessor. In this preferred embodiment, the output signals of the first and the second reference circuit, and hence the conduction times of the first and the second switching element, can be adjusted by means of the microprocessor.
Satisfactory results have also been obtained in examples of a switching device in accordance with the invention, wherein an input of the timer is coupled to the output of a reset circuit, and an input of the reset circuit is coupled to the microprocessor. In such examples, the reset circuit resets the timer with a frequency f, which can be set via the microprocessor.
To preclude that both switching elements become conducting at the same time, the control circuit of a switching device in accordance with the invention is preferably provided with a coupling-circuit part which is used for
maintaining, during a third time interval having a duration 1/(2*f), an electric connection between the output of the first comparator and the control electrode of the first switching element, and for interrupting an electric connection between the output of the second comparator and the control electrode of the second switching element, and for
maintaining, during a fourth time interval having a duration 1/(2*f), an electric connection between the output of the second comparator and the control electrode of the second switching element, and for interrupting an electric connection between the output of the first comparator and the control electrode of the first switching element. Said first time interval forms part of the third time interval and said second time interval forms part of the fourth time interval. In the third time interval, the first switching element is conducting during a time interval which is equal to the first time interval. During the remaining part of the third time interval, the first switching element is non-conducting. The second switching element is non-conducting during the whole third time interval. Correspondingly, in the fourth time interval, the second switching element is conducting during a time interval which is equal to the second time interval. During the remaining part of the fourth time interval, the second switching element is non-conducting. The first switching element is non-conducting during the whole fourth time interval. In an advantageous embodiment of such a coupling-circuit part, the coupling-circuit part comprises
a bistable multivibrator, an input of which is coupled to an output of the timer,
a first and gate, a first input of which is coupled to a first output of the bistable multivibrator, a second input of which is coupled to the output of the first comparator, and an output of which is coupled to the control electrode of the first switching element, and
a second and gate, a first input of which is coupled to a second output of the bistable multivibrator, a second input of which is coupled to the output of the second comparator, and an output of which is coupled to the control electrode of the second switching element.
In this manner, the coupling-circuit part is formed in a comparatively simple and reliable manner.
Preferably, the microprocessor, the timer, the first reference circuit, the second reference circuit and the coupling-circuit part of a control circuit of a switching device in accordance with the invention are integrated in an IC. As a result, the control circuit, and hence the switching device in accordance with the invention, can be embodied so as to be comparatively compact.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.